1. Field of the Invention
This invention is in the field of driver circuits for applying to a transmission line, the loading of which varies, a signal having a desired wave form, frequency, and peak to peak voltage.
2. Description of the Prior Art
The use of coupling transformers to permit a driver circuit to apply signals to a transmission line is known in the art. However, applicant is unaware of any prior art driver circuits which are capable of applying through a coupling transformer, signals on which strict limits are placed as to the wave form, peak to peak voltage, and frequency of the signal as applied to the transmission line.
The problem solved by the driver circuit of this invention is how to apply a relatively low frequency signal carrying binary coded information onto a transmission line which is simultaneously, or concurrently, carrying significantly higher frequency digital signals, without there being a conflict between the two signals.
The environment in which this invention is used is a token passing local area network in which a limited, but significant number of physical modules, the number of which can change from time to time, communicate with one another over the local control network, transmission line, bus, or coaxial cable. The communication between physical modules is at a frequency of 5.0 MHz in the preferred embodiment. The peak to peak voltage of the higher frequency signals is limited in the preferred embodiment to 800 MV. In this environment one physical module is permitted, or enabled, to transmit timing information over one of the coaxial cables of the local network bus. The timing information is included in a timing frame prepared by the timing subsystem of the physcial modules so enabled. The timing frame includes information which permits timing subsystems of all physical modules of the network to synchronize themselves with that of the transmitter. When a redundant cable is used, a second module may be enabled to transmit timing frames over the second cable at a lower frequency 12.5 K Hz in the preferred embodiment. A physical module which is capable of transmitting timing frames over the local control network cable is provided with driving circuit of this invention. For additional information on this environment, reference is made to concurrently filed patent application entitled Method and Apparatus for Synchronizing the Timing Subsystems of the Physical Modules of a Local Area Network by David L. Kirk, application Ser. No. 06/682,645 filed Dec. 17, 1984, which is assigned to Honeywell Inc., the assignee of this application. This disclosure of the above identified Kirk application is hereby incorporated by reference into this application.
The peak to peak voltage of the lower frequency signals applied to the cable or transmission line, has to be such that when it is added to the peak to peak voltage of the higher frequency signals, the sum of the two voltages is such as not to overdrive the receivers of the physical modules of the network. Controlling the peak to peak voltage is made more difficult because the load seen by the driver circuit is not necessarily constant. The wave form of the applied signal should not include harmonics of the higher frequency signals. If such harmonics were present, they could result in errors in the information transmitted at the higher frequency. In addition, the driver circuit has to provide high impedance to the higher frequency signals which are also carried by the cable to prevent overloading the driver circuit supplying the higher frequency signals.